Eddy current sensor arrays having drive windings with extended portions

ABSTRACT

An apparatus for the nondestructive measurements of materials. Eddy current sensing arrays are described which provide a capability for high resolution imaging of test materials and also a high probabilitity of detection for defects. These arrays incorporate layouts for the sensing elements which take advantage of microfabrication manufacturing capabilities for creating essentially identical sensor arrays, aligning sensing elements in proximity to the drive elements, and laying out conductive pathways that promote cancellation of undesired magnetic flux.

RELATED APPLICATION

[0001] This application is a divisional of U.S. application Ser. No.10/102,620 filed Mar. 19, 2002, which claims the benefit of U.S.Provisional Application No. 60/276,997 filed Mar. 19, 2001, the entireteachings of which are incorporated herein by reference.

BACKGROUND

[0002] The technical field of this invention is that of nondestructivematerials characterization, particularly quantitative, model-basedcharacterization of surface, near-surface, and bulk material conditionfor flat and curved parts or components using eddy-current sensors.Characterization of bulk material condition includes (1) measurement ofchanges in material state caused by fatigue damage, creep damage,thermal exposure, or plastic deformation; (2) assessment of residualstresses and applied loads; and (3) assessment of processing-relatedconditions, for example from shot peening, roll burnishing,thermal-spray coating, or heat treatment. It also includes measurementscharacterizing material, such as alloy type, and material states, suchas porosity and temperature. Characterization of surface andnear-surface conditions includes measurements of surface roughness,displacement or changes in relative position, coating thickness,temperature and coating condition. Each of these also includes detectionof electromagnetic property changes associated with single or multiplecracks. Spatially periodic field eddy-current sensors have been used tomeasure foil thickness, characterize coatings, and measure porosity, aswell as to measure property profiles as a function of depth into a part,as disclosed in U.S. Pat. Nos. 5,015,951 and 5,453,689.

[0003] Conventional eddy-current sensing involves the excitation of aconducting winding, the primary, with an electric current source ofprescribed frequency. This produces a time-varying magnetic field at thesame frequency, which in turn is detected with a sensing winding, thesecondary. The spatial distribution of the magnetic field and the fieldmeasured by the secondary is influenced by the proximity and physicalproperties (electrical conductivity and magnetic permeability) of nearbymaterials. When the sensor is intentionally placed in close proximity toa test material, the physical properties of the material can be deducedfrom measurements of the impedance between the primary and secondarywindings. Traditionally, scanning of eddy-current sensors across thematerial surface is then used to detect flaws, such as cracks.

[0004] In many inspection applications, large surface areas of amaterial need to be tested. This inspection can be accomplished with asingle sensor and a two-dimensional scanner over the material surface.However, use of a single sensor has disadvantages in that the scanningcan take an excessively long time and care must be taken whenregistering the measured values together to form a map or image of theproperties. These shortcomings can be overcome by using an array ofsensors or an array of elements within a single sensor, as described forexample in U.S. Pat. No. 5,793,206, since the material can be scanned ina shorter period of time and the measured responses from each arrayelement are spatially correlated. However, the use of arrays complicatesthe instrumentation used to determine the response of each arrayelement. For example, in one conventional method, as described forexample in U.S. Pat. No. 5,182,513, the response from each element of anarray is processed sequentially by using a multiplexer for each elementof the array. While this is generally faster than scanning a singlesensor element, there is still a significant time delay as theelectrical signal settles for each element and there is the potentialfor signal contamination from previously measured channels.

[0005] For nondestructive testing of conducting and/or magneticmaterials over wide areas, eddy current sensor arrays may be used. Theseeddy current sensors excite a conducting winding, the primary, with anelectrical current source of a prescribed frequency. This produces atime-varying magnetic field at the same frequency, which in turn isdetected with a sensing winding, the secondary. The spatial distributionof the magnetic field and the field measured by the secondary isinfluenced by the proximity and physical properties (electricalconductivity and magnetic permeability) of nearby materials. When thesensor is intentionally placed in close proximity to a test material,the physical properties of the material can be deduced from measurementsof the impedance between the primary and secondary windings.Traditionally, scanning of eddy-current sensors across the materialsurface is then used to detect flaws, such as cracks. When scanning overwide areas, these arrays may include several individual sensors, buteach sensor must be driven sequentially in order to prevent cross-talkor cross-contamination between the sensing elements.

[0006] Eddy current arrays have also been disclosed in U.S. Pat. No.5,262,722, however the implemented versions of these arrays usedifferential sensing elements. The use of differential sensing element,that essentially compare the response of two neighboring sensingregions, limits the capability to determine absolute properties ofinterest. These sensor arrays and conventional eddy current sensors arealso highly sensitive to sensor position, requiring expensive automatedscanners to build images of material properties for complex surfaceinspections. Differential sensors may also produce false indications onrelatively rough surfaces, such as surfaces with fretting damage.

SUMMARY

[0007] Aspects of the inventions described herein involve novel sensorsfor the measurement of the near surface properties of conducting and/ormagnetic materials. These sensors use novel geometries for the primarywinding and sensing elements that promote accurate modeling of theresponse and provide enhanced capabilities for the creation of images ofthe properties of a test material.

[0008] In one embodiment, sensor array designs are disclosed that permitthe creation of property images when scanned over a material surface. Inone embodiment, the drive winding includes at least one centralconducting segment and parallel return segments located on either sideto impose a periodic magnetic field of at least two spatial wavelengthsin a test material, a linear array of sensing elements to sense theresponse to the test material properties, and at least one sensingelement uses a magnetoresistive (MR) or giant magnetoresistive (GMR)sensor. Secondary coils can also be placed around one or more of the MRor GMR sense elements, in one embodiment. In another, these coils areconnected in a feedback configuration, and, in one embodiment, act tomaintain the magnetic field at the MR or GMR sensor at a prescribedlevel.

[0009] In another embodiment of a sensor array design, the drive windingincludes at least one central conducting segment and at least oneparallel return segment on either side, a linear array of sensingelements between the central segments and a return segment, and separateconnections to each sensing element. The distance between the centralsegments and the return segments can be selected to align with featuresof interest in a test material, such as bolt holes. One embodimentincludes two central conductors and a return path for each conductor,with equal distances between the central conductors and each returnpath. In another embodiment, a second linear array of sensing elementsis placed between another pair of linear drive winding segments,parallel to the first linear array. In one form, each element in thefirst array is aligned with an element in the second array. In anotherform, elements in the first array are offset from the elements in thesecond array in a direction parallel to the linear drive windingsegments. Preferably, this offset distance is one-half of the length ofa sense element, which ensures complete coverage of the element in adirection perpendicular to the drive winding segments. In an embodiment,the linear arrays are equally distant from the central conductors.Differential measurements may also be taken in the response betweenelements in the first array and elements in the second array. Thecentral conductors can be placed in the same plane as the sensingelements to improve the coupling with the sense elements.

[0010] In an embodiment, the conductivity and proximity of the sensingelements to the surface are measured to detect cracks. In another, theproximity of each sensing element to the test material surface is usedto determine surface roughness. In another embodiment, the sensingelement response is used for health monitoring or condition assessmentof a component. An embodiment also includes the use of a characteristicsensor response for a flaw and using that characteristic response toconstruct a filter. This filter can be applied to a response image toemphasize indications that are likely to be associated with flaws andsuppresses indications unlikely to be associated with flaws.

[0011] In one embodiment, a single encoder determines the position ofthe array while scanning. In another embodiment, an automated scanner isused to move the array over a test material. In another embodiment,using modular fixtures with position encoders facilitates manualscanning of complex parts. In an embodiment, a template is used to alignincremental scans over a test material so that images of the materialproperties over areas wider than the array width can be generated.

[0012] To facilitate the scanning of a sensor array over a material testsurface, another embodiment includes the use of a fluid filled balloon.In an embodiment, this balloon is attached to a shuttle and the shuttleis shaped to approximately match the shape of the test material. Inanother embodiment, the sensor and balloon components are modularizedand can be replaced rapidly. In one embodiment, the inspection isperformed on the surface of a bolt hole. In another, the inspection isperformed on the inside of an engine disk slot.

[0013] In another embodiment of a sensor array design, the drive windingincludes at least one pair of parallel conducting segments to impose amagnetic field in the test material, a linear array of sensing elements,and separate connections to each sensing element. The distance betweenthe parallel segments can be selected to align with features of interestin a test material. In one form, the linear array is placed between theparallel segments of the drive. Preferably, in another form, the arrayis placed outside the loop formed by the parallel segments of the drive.This also permits both the drive segments and the sense elements to beplaced in the same plane.

[0014] In another embodiment, a second linear array of sensing elementsis placed parallel to the first linear array of elements. This secondarray can be placed between the parallel segments of the drive winding,near a segment of the drive winding common with the first array. Inanother embodiment, the second array is placed outside the drive windingloop, opposite that of the first array. The distances between the lineararrays and the drive winding segments can be selected to be the same ordifferent. In one embodiment, each element in the first array is alignedwith an element in the second array. In another embodiment, elements inthe first array are offset from the elements in the second array in adirection parallel to the linear drive winding segments. Preferably,this offset distance is one-half of the length of a sense element.

[0015] In an embodiment for the sensor array, the locations of thesensing elements in a direction parallel to the drive segments and thesensing element size can be made non-uniform to provide a higher imageresolution over specific material test areas. In another embodiment, thesensor array can be fabricated onto a flexible substrate so that thesensor can conform to the shape of the test material. Alternatively, thesensor array can be fabricated onto a rigid substrate. With eithersubstrate material, measurements of the material properties can beperformed in a noncontact fashion. In an embodiment, at least one of thesensing elements includes a MR or GMR sensor. In one form, these sensingelements also include a secondary coil. In another form, the secondarycoil is used in a feedback configuration.

[0016] In yet another embodiment, a sensor array design comprises twoparallel linear rows of sensing elements on opposite sides of a centralconductor for detecting cracks on each side of a feature. In oneembodiment this feature is a fastener in an aircraft skin. In anotherembodiment, multiple frequency measurements are used to removeinterference cause by the feature itself to isolate and emphasize theresponse of the crack. An embodiment also includes using the sensorresponse from a sensing element to create a characteristic response fora flaw and to construct a filter. This filter can be applied to aresponse image to emphasize indications that are likely to be associatedwith flaws and suppresses indications unlikely to be associated withflaws. In one form, the flaw is a crack. In another, the flaw is aburied inclusion.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of preferred embodiments of the invention, as illustrated inthe accompanying drawings in which like reference characters refer tothe same parts throughout the different views. The drawings are notnecessarily to scale, emphasis instead being placed upon illustratingthe principles of the invention.

[0018]FIG. 1 is a drawing of a spatially periodic eddy current sensorarray.

[0019]FIG. 2 is an expanded view of the drive and sense elements for thespatially periodic eddy current sensor array shown in FIG. 1.

[0020]FIG. 3 is a pictorial cross-sectional view of some of the driveand sense elements for the sensor of FIG. 1.

[0021]FIG. 4 is a plot of the calculated sensor response to a notch asthe gap between the sensing elements and the central primary conductorsis varied.

[0022]FIG. 5 is an unfiltered measurement image taken with an eddycurrent sensing array over a Titanium alloy plate containing cracks at afrequency of 8 MHz.

[0023]FIG. 6 is an unfiltered measurement image taken with an eddycurrent sensing array over a Titanium alloy plate containing cracks at afrequency of 12 MHz.

[0024]FIG. 7 is a filtered measurement image that combines the data ofFIG. 5 and FIG. 6 to highlight the cracks.

[0025]FIG. 8 is a plot of the unfiltered 8 MHz sensor response fromelement 7 in the trailing row of elements in the array used to scan overa Titanium alloy plate containing cracks.

[0026]FIG. 9 is a plot of the 8 MHz sensor response to a single crack ina Titanium plate.

[0027]FIG. 10 is a plot of the filtered sensor response from element 7using the shape responses like those of FIG. 9 and both measurementfrequencies.

[0028]FIG. 11 is a drawing of a spatially periodic field eddy currentsensor array having all connection leads on one side of the array.

[0029]FIG. 12 is a drawing of an eddy current sensing array being nearan opening in a test material.

[0030]FIG. 13 is a drawing of an eddy current sensing array beinginserted into a pipe.

[0031]FIG. 14 is a cross-sectional view of an eddy current sensing arrayinside a pipe.

[0032]FIG. 15 is another drawing of an eddy current sensing array beingnear an opening in a test material.

[0033]FIG. 16 is another drawing of an eddy current sensing array beinginserted into a pipe.

[0034]FIG. 17 is a drawing of a single wavelength eddy current sensorarray.

[0035]FIG. 18 is an expanded view of the drive and sense elements forthe eddy current array shown in FIG. 17.

[0036]FIG. 19 is a pictorial cross-sectional view of the drive and someof the sense elements for the eddy current array shown in FIG. 17.

[0037]FIG. 20 is an expanded view of the drive and sense elements for aneddy current array having offset rows of sensing elements.

[0038]FIG. 21 is a plot of the calculated response to a surface breakingnotch using a model, indicating the response to the secondary element onthe left side of the central conductor.

[0039]FIG. 22 is an expanded view of the drive and sense elements for aneddy current array having a single row of sensing elements.

[0040]FIG. 23 is a schematic of the normalized conductivity for ameasurement channel of a high-resolution MWM-Array with longer segmentsof the primary winding oriented parallel to the weld axis for a similarmetal zero LOP defect specimen.

[0041]FIG. 24 is an expanded view of an eddy current array where thelocations of the sensing elements along the array are staggered.

[0042]FIG. 25 is an expanded view of an eddy current array with a singlerectangular loop drive winding and a linear row of sense elements on theoutside of the extended portion of the loop.

[0043]FIG. 26 is a schematic for an eddy current array with a singlerectangular loop drive winding and two rows of sense elements on theoutside of the extended portions.

[0044]FIG. 27 is a schematic for an eddy current array with differentdistances between each row of sensing elements and the drive winding.

[0045]FIG. 28 is a schematic for an eddy current array with a spatialoffset between each row of sensing elements, parallel to the extendedportions of the drive winding.

[0046]FIG. 29 is a schematic for an eddy current array having a row ofsensing elements inside the drive winding loop and a row of sensingelements outside the drive winding loop.

[0047]FIG. 30 is a schematic for an eddy current array with anelectronic circuit at one end of the primary winding loop.

[0048]FIG. 31 shows a conductivity image for an aluminum bending fatiguespecimen obtained from an MWM-Array scanned with the array driveperpendicular to the specimen axis.

[0049]FIG. 32 shows a conductivity image for an aluminum bending fatiguespecimen obtained from an MWM-Array scanned with the array driveparallel to the specimen axis.

[0050]FIG. 33 shows the conductivity/lift-off measurement grid used toproduce data in FIG. 32.

[0051]FIG. 34 shows a plate thickness image for a floor chine plateobtained with an MWM-Array and a thickness/lift-off measurement grid.

[0052]FIG. 35 shows another plate thickness image for a floor chineplate obtained with an MWM-Array and a thickness/lift-off measurementgrid.

[0053]FIG. 36 shows another thickness image of the same data as in FIG.35 with the image scale highlighting low to intermediate corrosion lossregions.

[0054]FIG. 37 shows MWM-Array generated images of the 5 percent maximummaterial loss, represented by a dome-shaped cavity on the inside firstlayer surface (left image) between two 0.04-in. thick aluminum skins;inside second layer surface (right image) between two 0.04-in. thickaluminum skins.

[0055]FIG. 38 shows a measurement grid and responses of a single channelof an MWM-Array to material loss between two layers as the sense elementis scanned across the loss region for first layer thinning.

[0056]FIG. 39 shows a measurement grid and responses of a single channelof an MWM-Array to material loss between two layers as the sense elementis scanned across the loss region for second layer thinning.

[0057]FIG. 40 shows a plot of first and second layer material loss forindividual MWM sensing elements scanned across the maximum loss pointfor reported 5 percent and 10 percent material loss.

[0058]FIG. 41 shows a normalized permeability image obtained from anMWM-Array scanned over a double-notched 4340 low-alloy steel tensilespecimen after failure in a tension test, with an excitation frequencyof 1 MHz and the extended portions of the primary winding orientedparallel to the loading axis.

[0059]FIG. 42 shows a normalized permeability image obtained from anMWM-Array scanned over a double-notched 4340 low-alloy steel tensilespecimen after failure in a tension test, with an excitation frequencyof 158 kHz and the extended portions of the primary winding orientedparallel to the loading axis.

[0060]FIG. 43 shows a normalized permeability image obtained from anMWM-Array scanned over a double-notched 4340 low-alloy steel tensilespecimen after failure in a tension test, with an excitation frequencyof 1 MHz and the extended portions of the primary winding orientedperpendicular to the loading axis.

[0061]FIG. 44 shows a normalized permeability image obtained from anMWM-Array scanned over a double-notched 4340 low-alloy steel tensilespecimen after failure in a tension test, with an excitation frequencyof 158 kHz and the extended portions of the primary winding orientedperpendicular to the loading axis.

[0062]FIG. 45 shows a permeability/lift-off measurement grid and datafrom a single element of an MWM-Array.

[0063]FIG. 46 shows a schematic of MWM-Rosette designed for detection ofcracks at fasteners.

[0064]FIG. 47 shows a linear MWM-Array used to monitor crack initiationand growth along a linear feature.

[0065]FIG. 48 shows a linear MWM-Array used to detect cracks thatpropagate across a specific location within a structural member.

[0066]FIG. 49 shows data from a fatigue test with an MWM-Rosette mountedaround a hole in an aluminum dogbone specimen, the test being stoppedshortly after the crack reached channel 6.

[0067]FIG. 50 shows the crack size vs. number of load cycles based onthe test data shown in FIG. 49.

[0068]FIG. 51 shows the structure of a rotationally symmetric shapedfield drive winding.

[0069]FIG. 52 shows results of conductivity/lift-off measurements withthe circular magnetometer.

[0070]FIG. 53 shows an area scan of a stainless steel plate with thecrack at the surface.

[0071]FIG. 54 shows the structure of the hybrid sensor feedback loop.

[0072]FIG. 55 shows multiple GMR sensors placed within a feedback coiland at the center of a drive winding.

[0073]FIG. 56 shows multiple GMR sensors placed within a feedback coiland offset near an edge of a drive winding.

[0074]FIG. 57 shows two linear arrays of GMR sensors placed withinfeedback coils and external to the drive winding.

DETAILED DESCRIPTION OF THE INVENTION

[0075] A description of preferred embodiments of the invention follows.The design and use of high resolution conformable eddy current sensorarrays is described for the nondestructive characterization ofmaterials. These sensor arrays are well suited to inspections over wideareas as a single scan of the sensor array allows the materialproperties to be determined over a relatively wide distance. Also,sequential scans can be concatenated, with or without overlap, to createimages over wide areas. Furthermore, simple manual scans can be usedwith only a roller encoder to record position, still producingtwo-dimensional images of the quality previously achieved with high costautomated scanners. Measurements of the responses from each element in alinear array of sensing elements, oriented perpendicular to the scandirection, also facilitates the creation of material property images sothat the presence of property variations or defects are readilyapparent.

[0076] In one embodiment, eddy current sensor arrays with at least onemeandering drive winding and multiple sensing elements are used toinspect the test material. An example sensor array is shown in FIG. 1.Expanded views of the region near the sensing elements are shown in FIG.2 and FIG. 3. This array includes a spatially periodic primary winding70 having extended portions for creating the magnetic field and aplurality of secondary elements 72 within the primary winding forsensing the response to the material under test (MUT). The primarywinding is fabricated in a periodic pattern with the dimension of thespatial periodicity termed the spatial wavelength λ. This geometry canbe described as a meandering winding so that a single element sensor,where all of the sensing elements are connected together, can be calleda Meandering Winding Magnetometer (MWM®) and a sensor array having asimilar primary winding an MWM-Array, as described in U.S. patentapplication Ser. No. 10/010,062, filed Nov. 13, 2001, the entireteachings of which are incorporated herein by reference. Melcher firstconceived the use of meandering or rectangular drives with multiplesensing regions and drive wires connected in series to cover asignificant area, as described in U.S. Pat. No. 5,015,951. Detaileddescriptions of this geometry for an eddy current sensor are given inU.S. Pat. Nos. 5,453,689, 5,793,206, and 6,188,218. In U.S. Pat. No.5,262,722, a similar approach to Melcher's is used to link seriesconnected drive regions to excite differential sensing elements. In theMWM sensors, a time-varying current is applied to the primary winding,which creates a magnetic field that penetrates into the MUT and inducesa voltage at the terminals of the secondary elements. This terminalvoltage reflects the properties of the MUT. The secondary elements arepulled back from the connecting portions of the primary winding tominimize end effect coupling of the magnetic field. Dummy elements 74can be placed between the meanders of the primary to maintain thesymmetry of the magnetic field, as described in U.S. Pat. No. 6,188,218.The magnetic vector potential produced by the current in the primary canbe accurately modeled as a Fourier series summation of spatialsinusoids, with the dominant mode having the spatial wavelength λ. Foran MWM-Array, the responses from individual or combinations of thesecondary windings can be used to provide a plurality of sense signalsfor a single primary winding construct as described in U.S. Pat. Nos.5,793,206 and Re. 36,986 and also U.S. application Ser. No. 09/666,879,filed Sep. 20, 2000, the entire teachings of which are incorporatedherein by reference, and U.S. application Ser. No. 09/666,524, filedSep. 20, 2000, now U.S. Pat. No. 6,657,429, the entire teachings ofwhich are incorporated herein by reference.

[0077] The MWM structure can be produced using micro-fabricationtechniques typically employed in integrated circuit and flexible circuitmanufacture. This results in highly reliable and highly repeatable(i.e., essentially identical) sensors, which has inherent advantagesover the coils used in conventional eddy-current sensors which exhibitsignificant sensor-to-sensor variability even for nominally identicalsensors. As indicated by Auld and Moulder, for conventional eddy-currentsensors “nominally identical probes have been found to give signals thatdiffer by as much as 35%, even though the probe inductances wereidentical to better than 2%” (Auld, 1999). The lack of reproducibilitywith conventional coils introduces severe requirements for calibrationof the sensors (e.g., matched sensor/calibration block sets). Incontrast, duplicate MWM sensor tips have nearly identical magnetic fielddistributions around the windings as standard micro-fabrication(etching) techniques have both high spatial reproducibility andresolution. As the sensor was also designed to produce a spatiallyperiodic magnetic field in the MUT, the sensor response can beaccurately modeled which dramatically reduces calibration requirements.For example, in some situations an “air calibration” can be used tomeasure an absolute electrical conductivity without calibrationstandards, which makes the MWM sensor geometry well-suited to surfacemounted or embedded applications where calibration requirements will benecessarily relaxed.

[0078] An efficient method for converting the response of the MWM sensorinto material or geometric properties is to use grid measurementmethods. These methods map the magnitude and phase of the sensorimpedance into the properties to be determined and provide for areal-time measurement capability. The measurement grids aretwo-dimensional databases that can be visualized as “grids” that relatetwo measured parameters to two unknowns, such as the conductivity andlift-off (where lift-off is defined as the proximity of the MUT to theplane of the MWM windings). For the characterization of coatings orsurface layer properties, three-dimensional versions of the measurementgrids can be used. Alternatively, the surface layer parameters can bedetermined from numerical algorithms that minimize the least-squareserror between the measurements and the predicted responses from thesensor.

[0079] An advantage of the measurement grid method is that it allows forreal-time measurements of the absolute electrical properties of thematerial. The database of the sensor responses can be generated prior tothe data acquisition on the part itself, so that only table lookupoperation, which is relatively fast, needs to be performed. Furthermore,grids can be generated for the individual elements in an array so thateach individual element can be lift-off compensated to provide absoluteproperty measurements, such as the electrical conductivity. This againreduces the need for extensive calibration standards. In contrast,conventional eddy-current methods that use empirical correlation tablesthat relate the amplitude and phase of a lift-off compensated signal toparameters or properties of interest, such as crack size or hardness,require extensive calibrations and instrument preparation.

[0080] While a single meandering conductor can be used for the primarywinding, this leads to the formation of a large inductive loop that caninfluence the eddy current sensor response. Splitting the primarywinding so that the return leads to each component of the drive windingare in close proximity to one another can substantially reduce theeffects of this extraneous inductive loop. In FIG. 1, FIG. 2 and FIG. 3,the primary winding 70 is split into two parts so that each extendedportion of a primary winding meander, except for the endmost, includestwo conducting elements. Each loop of the primary winding is connectedtogether in series and the primary windings are wound so that thecurrent through adjacent conductors is in the same direction. Thecurrent through these two conducting loops imposes a spatially periodicmagnetic field. This winding configuration minimizes the effects ofstray magnetic fields from the lead connections to the primary winding,which can create an extraneous large inductive loop that influences themeasurements, maintains the meandering winding pattern for the primary,and effectively doubles the current through the extended portions of themeanders. This method for reducing the effects of the extraneous loop isdescribed more completely in U.S. application Ser. No. 09/666,879, nowU.S. Pat. No. 6,657,429, and Ser. No. 09/666,524.

[0081] When the sensor is scanned across a part or when a crackpropagates across the sensor, perpendicular to the extended portions ofthe primary winding, secondary elements 76 in a primary winding loopadjacent to the first array of sense elements 72 provide a complementarymeasurement of the part properties. These arrays of secondary elements76 are aligned with the first array of elements 72 so that images of thematerial properties will be duplicated by the second array.Alternatively, to provide complete coverage when the sensor is scannedacross a part the sensing elements 76 can be offset along the length ofthe primary loop or when a crack propagates across the sensor,perpendicular to the extended portions of the primary winding, secondaryelements 76 in a primary winding loop adjacent to the first array ofsense elements 72 can be offset along the length of the primary loop, asillustrated in FIG. 20. Additional primary winding meander loops, whichonly contain dummy elements, are placed at the edges of the sensor tohelp maintain the periodicity of the magnetic field. The connectionleads 78 to the secondary elements are perpendicular to the extendedportions of the primary winding, which necessitates the use of amulti-layer structure in fabricating the sensor. The layers that containthe primary and secondary winding conductors are separated by a layer ofinsulation. Layers of insulation are generally also applied to the topand bottom surfaces of the sensor to electrically insulate the primaryand secondary windings from the MUT. A protective layer is alsosometimes used, e.g. Kapton™ or Teflon™ with or without a removableadhesive. This layer becomes sacrificial, protecting the sensor andbeing periodically removed and replaced with age.

[0082] The leads to the primary and secondary elements are kept closetogether to minimize fringing field coupling. The leads 82 for theprimary winding are kept close together to minimize the creation offringing fields. The leads 78 for the secondary elements are kept closetogether to minimize the linkage of stray magnetic flux. The bond pads86 provide the capability for connecting the sensor to a mountingfixture. The bond pads 86 are spread out for easier design, contact, andassembly of the connectors to the bond pads. The trace widths for theprimary winding can also be increased to minimize ohmic heating,particularly for large penetration depths that require low frequency andhigh current amplitude excitations. Also, the conducting primarythickness and width may be increased to minimize ohmic heating.

[0083] The placement of the sensing elements near the primary windingscan also be adjusted to enhance sensitivity to specific types of flawsor defects. FIG. 2 shows an expanded view around the sensing elementsfor the sensing array of FIG. 1. The arrays of sensing elements 72 and76 are located relatively close to the common portions of the primarywinding 70 so that the distance 80 is smaller than the distance 81. PastMWM designs emphasized placing the sensing elements at the center of thegap between the primary winding legs, so that the distances 80 and 81were equal, to minimize coupling of short spatial wavelength magneticfield modes. With the previous design, scanning the sensor array over asmall (compared to a half-wavelength) surface breaking or near-surfacedefect leads to a double-humped response from the sensing element, witheach hump occurring when the drive windings nearest to the sensingelements are predominantly over the defect. Physically, for a conductingMUT, the time varying magnetic field induces eddy currents in the MUTthat mirror the conductor pattern of the primary winding and theseinduced eddy currents are largest beneath the primary windings. Thepresence of a defect interrupts this current flow and the perturbationsin the magnetic field are detected with the sensing elements. With thenew offset secondary design (distances 80 and 81 unequal), the responseto the defect will be asymmetric with an enhanced response when thedefect is beneath the common or central primary winding 70 and a reducedresponse when the defect is beneath the further or return primarywindings. FIG. 4 illustrates this effect. Modeling was performed tocalculate the response in the relative phase change as a flaw, in thiscase a rectangular notch, passes beneath a single element of an array.Increasing the gap between the central conductor and the return windingcauses a decrease in the response peaks for flaw locations beneath thecentral primary conductor and the secondaries but an increase in theresponse peak for flaw locations beneath the return winding. Theresponse when the defect is beneath the return portion of the primarywinding can be reduced by moving the return winding further away fromthe sensing elements, as shown in FIG. 21. Reducing the response fromthe return is an advantage when trying to build images and improvereliability.

[0084] The arrays of sensing elements 72 and 76 are offset the samedistance 80 from the common primary winding 71 to maintain thecapability for obtaining the same measurement from a given defect.Material property variations and orientation of non-spherical defectscan affect the responses of the sensing elements in each arraydifferently, which provides the potential to separate defect featuresand defect signals from background property variations. A simple filterwould be to sum the responses of spatially correlated sensing elements(when the scanning direction is perpendicular to the extended portionsof the primary winding), which would highlight the presence of defectswhen underneath the common drive winding 71. Furthermore, filters cancompare the sensing element responses to ensure that the spatiallycorrelated sensing elements are responding to the same feature. Inaddition, the responses of neighboring sensing elements can be used tonormalize the response, eliminating background variations of thematerial properties. Multiple frequency measurements can also be used toenhance the results.

[0085] The distinct shapes of the sensor response when passing over aflaw can be isolated using “matched filters” as described in applicationSer. No. 10/010,062, now abandoned. Then, by searching an image for thisdistinctive shape the response to a local defect can be enhanced. As anexample, this process is illustrated in FIG. 5 through FIG. 10 forcracks in a Titanium alloy flat crack standard. Unfiltered images of theeffective conductivity images from a scan over the standard are shown inFIG. 5 for an 8 MHz excitation and in FIG. 6 for a 12 MHz excitation. Onthis particular standard, there are three cracks of lengths 0.711,0.635, and 0.686 mm (0.028, 0.025, and 0.027-inches, respectively) alongthe path of element 7. A filtered image, shown in FIG. 7, hashighlighted cracks and suppressed background noise variations.

[0086] In this case the filtered image combines the response from boththe trailing and leading rows of sensing elements at both measurementfrequencies into a single response. This is accomplished for each row(trailing and leading) and measurement frequency (8 MHz and 12 MHz) byfirst calculating, element-by-element, the correlator of a moving windowof data with a shape filter. For example FIG. 8 shows the unfiltered 8MHz data from element 7 in the trailing row and FIG. 9 shows thetrailing row shape response. Similar responses are used for the leadingrow data and the 12 MHz data. The resulting signal can be denotedx₁(i,j) where the index i denotes element number and j denotes themeasurement number. Repeating this process yields x₂(i,j) for the 8 MHzleading row data, y₁(i,j) for the 12 MHz trailing row data, and y₂(i,j)for the 12 MHz leading row data. The results from each row of the 8 MHzdata are then combined as${x\left( {i,j} \right)} = \frac{\left( {{x_{1}\left( {i,j} \right)} + {x_{2}\left( {i,j} \right)}} \right)}{2*2^{({{x_{1}{({i,j})}} - {x_{2}{({i,j})}}})}}$

[0087] and the results from each row of the 12 MHz data are thencombined as${y\left( {i,j} \right)} = \frac{\left( {{y_{1}\left( {i,j} \right)} + {y_{2}\left( {i,j} \right)}} \right)}{2*2^{({{y_{1}{({i,j})}} - {y_{2}{({i,j})}}})}}$

[0088] Then, the results from each frequency are combined as${z\left( {i,j} \right)} = \frac{\left( {{x\left( {i,j} \right)} + {y\left( {i,j} \right)}} \right)}{2*2^{({{x{({i,j})}} - {y{({i,j})}}})}}$

[0089] This result is shown in FIG. 10 for element 7 alone and in FIG. 7for all of the elements. Note that this particular procedure suppressessignals on one row of elements but not the other row, at a givenfrequency. It also suppresses signals that appear on only one frequencybut not the other. This improves clutter suppression to limit falsealarms.

[0090]FIG. 2 and FIG. 3 also show that the connection leads 83 to eachsensing element are closely paralleled by another set of leads 85 endingin a closed loop 87. As described in U.S. application Ser. No.09/666,879, now U.S. Pat. No. 6,657,429, and Ser. No. 09/666,524, thedifferential response between the actual sensing element and theparallel leads 85 is measured. This “flux cancellation” configurationprovides a measure of the absolute signal in the vicinity of the sensingelement and helps to minimize the effects of stray inductive andcapacitive coupling to the sensing element leads. The use of fluxcancellation allows longer lead lines to be used, permits the spreadingout of connection leads 83 so that standard pins can be used for theconnections and eliminates cross-talk problems encountered in closelypacked connection schemes, and also allows the sensor part of the probeto incorporate a connection board. The elimination of tightly packedconnectors is a significant cost and durability advantage. Furthermore,this use of a differential measurement to obtain absolute signalresponses from the sensing elements permits calibration in air, wherecalibration coefficients are obtained from comparisons of the sensorsignal in air to the predicted response for the sensor based on a modelfor the sensor geometry. This then permits an absolute measurement ofthe electromagnetic and geometric properties of the MUT, such aselectrical conductivity, magnetic permeability and layer thickness)without the use of calibration standards. Of course, the sensor orsensor arrays can also be calibrated on reference standard having knownproperties. In contrast, the use of conventional differential andabsolute eddy current sensors requires performing calibrationmeasurements on reference standards to set the gain levels for thisinstrumentation before quantitative MUT property information can beobtained. In this design the primary windings 70 are separated from thesecondary element arrays 72 and 76 by a layer of insulation 95. Thislayer of insulation is typically 0.5 to 1 mil (12.7 to 25.4 micrometers)thick Kapton™.

[0091]FIG. 11 shows another configuration for an MWM-Array. In this caseall of the leads 78 to the sensing element arrays and the leads 82 tothe primary winding are on one side of the sensor. This allows theactive area of the sensor, defined by the area covered by the primarywindings 70 to be inserted into confined areas such as bolt holes ordisk slots. Scanning of the array in a direction perpendicular to theextended portions of the primary winding, which could require rotatingthe sensor in a bolt hole, allows complete coverage of the inspectionarea. The sensor might also be oriented with the extended portions ofthe primary windings at a right angle to that shown or as shown with thesensing region offset relative to the connector to permit insertion intogeometric features such as the inside surface of pipes, bolt holes, andgun barrels, which is illustrated in FIG. 12 and FIG. 14.

[0092] In FIG. 12, the array 132 has a sensing array 130 offset from theconnector (with numerous bond pads) parallel to the direction of thelongest dimension of the connector. The extended portions of the primarywinding for creating the magnetic field are parallel to the offsetdirection of the array. Two arrays of sensing elements are placed at thecenter of and run parallel to the extended portions of the primarywinding. The sensing array is fabricated onto a flexible Kapton™ liningor substrate 136, which permits the shape of the sensing structure to bedeformed for insertion of the eddy current sensing array into confinedareas 134 such as pipes and bolt holes to inspect for defects anddamage. The conformability of the sensing array inside the confinedspace is illustrated in FIG. 13. FIG. 14 shows a cross-sectional view ofthe sensor inside this space. The sensor 132 can be held against theinside surface 142 of the test material 134 with a foam or balloonsupport 140. This support provides both a reasonably rigid framework forholding the sensing structure in-place when inserted into the hole andalso a compliant backing for maintaining intimate contact between thesensing structure and the inside surface of the test material. Rotatingthe sensing structure in a circumferential direction, perpendicular tothe extended portions of the primary winding, then permits completecoverage of the surface of the test material during the inspection. Theinspection can also be performed with the sensing structure placed atdifferent distances into the pipe so that pipe lengths greater than thelength spanned by the sensing element arrays can be inspected. This alsoallows multiple measurements of a given area to be performed. Thedistance into the pipe can be monitored with a position encoder orcontrolled with a robotic arm to permit accurate measurements of theinsertion distance into the pipe.

[0093]FIG. 15 shows another embodiment for the inspection of confinedareas such as a pipe. In this case the extended portions of the primarywinding, and the arrays of sensing elements 138, are orientedperpendicular to the offset direction of the sensing structure from theconnector. Again, the sensing structure is mounted onto a foam orballoon substrate which allows the sensing structure to conform to thesurface of the test material when inserted into a pipe or other confinedspace, as illustrated in FIG. 16. Here, moving the sensing structure inan axial direction, perpendicular to the extended portions of theprimary winding, then permits complete coverage of the surface of thetest material under the sensing elements during the inspection.Measurement scans at different angular positions along the circumferenceof the hole can then provide a complete inspection of the entire holesurface.

[0094]FIG. 17 shows another embodiment for an MWM-Array, with anexpanded view of the primary meanders and the sensing elements in FIG.18 and FIG. 19. In this case, the number of primary winding meanders 91is reduced so that measurements can be performed closer to materialedges without affecting the sensor response. The primary conductors 91of FIG. 17 and FIG. 18 show a single wavelength for a primary windingmeander. The secondary element arrays 72 and 76 are brought close to thecentral conductors of the primary 71, so that the gap 80 between theextended portions of the primary and secondary windings is smaller thanthe gap 81. In this region, the magnetic field distribution is similarto the spatially periodic magnetic field distribution of a primarywinding having more than one meander. As described in U.S. applicationSer. No. 09/666,879, now U.S. Pat. No. 6,657,429, and Ser. No.09/666,524, as well as U.S. application Ser. No. 09/891,091, filed Jun.25, 2001, now abandoned, the entire teachings of which are incorporatedherein by reference, this structure still has the leads for the primaryand the secondary close to one another and the split primary windingdesign has two conductors in the central region 71 which also eliminatesthe presence of large, extraneous external loops for linking magneticflux.

[0095] To help reduce the series resistance for the connection leads 78and 82 the conductors are made wider in regions 93 far from the sensingregion determined by the extended portions of the primary winding 91.This reduction in series resistance reduces the ohmic heating of theprimary winding when driven by the alternating current.

[0096] Reducing the number of extended portions of the primary windingmeanders has several advantages. First, since sensing elements arecloser to the endmost primary winding conductors, measurements can beperformed closer to the edge of a material before extended portions ofthe primary winding go off the material edge and affect the measuredsignal. Second, the inductance of the primary winding circuit or thedrive impedance also decreases so that it is easier to drive currentthrough the primary, at a given voltage, at high frequencies such as 10to 30 MHz. Third, the sensing element leads 83 cross-over a smallernumber of primary winding conductors, which, in addition to the use ofthe parallel conducting loops 85, reduces the susceptibility toelectrical noise and undesired, stray magnetic flux distant from thesensing element. The capability to measure at higher frequenciescombined with the flux cancellation lead design (83, 85, 87) permit useof smaller sensing elements with low noise instrumentation, as describedin U.S. application Ser. No. 10/010,062. These smaller elements (1)improve sensitivity to small defects, (2) increase the resolution forimaging internal geometric features, such as cooling holes, corrosion orpitting, (3) reduce edge effects, (4) improve surface topology mappingcapabilities, and (5) improve coverage and quality in imaging thequality of processes such as shot peening, coating thickness andporosity, case hardening, and grinding.

[0097] Another feature illustrated in FIG. 19 is that the centralportion of the primary winding 71 and the arrays of sensing elements 72and 76 lie in the same plane. The return legs for the primary winding 97are on a different plane and connected to the central portion conductors71 with vias at the ends of the primary winding half-meanders. Thisallows for direct connections to the sensing elements with a minimumnumber of vias, which improves both reliability and manufacturability ata reasonable cost. Placing the critical portions of the sensor, thecentral portion of the primary winding and the secondary elements, onthe same plane also allows higher precision fabrication processes to beused. For example, standard fabrication techniques have placementtolerances between copper paths on the same layer of 3 mils (75micrometers). In contrast, the layer to layer alignment tolerance forcopper paths is normally up to 5 mils (125 micrometers). This improvesthe manufacturing reproducibility of the sensor array. Placing thecentral portion of the primary windings and the secondary elements onthe same plane also provides enhanced sensitivity to cracks and defects.One reason is that the distance between the primary and the secondaryelements is smaller than when the primary windings are in the backplane, which increases the inductive coupling between the primary andthe secondary. Another reason is that the eddy currents induced by theapplied field are larger when the primary is closer to the MUT.

[0098] In a similar fashion, the central portion of the primary windingcould also be placed in the same plane as the secondary elements for thearrays having more than one meandering, as in FIG. 1 and FIG. 11. Thiswould provide the benefit of increased sensitivity to defects and onlyrequire via connections at the ends of the central primary windings. Theremaining extended portions of the primary windings can not be in theplane of the secondary elements because they would interfere with thelayout or pathways for the connection leads to the sensing elements.

[0099] In another embodiment, the linear rows of sensing elements can beoffset from one another, as shown in FIG. 20, so that scanning of thearray in a direction perpendicular to the sensing elements ensurescomplete coverage of the MUT and no defects are missed in the gapsbetween sensing elements. The drive on this array comprises two loopshaving extended portions and connected in series so that the current ineach of the conductors 71 in the center of the drive is in the samedirection.

[0100] The effective spatial wavelength or the distance between thecentral conductors 71 and the current return conductor 91 can be alteredto adjust the sensitivity of a measurement for a particular inspection.For example, a sensor array such as FIG. 20 can be scanned over thesurface of an MUT to inspect for surface breaking flaws or flaws hiddenbeneath material layers. For the sensor array of FIG. 20, the distance80 between the secondary elements 72 and the central conductors 71 issmaller than the distance 81 between the sensing elements 72 and thereturn conductor 91. Modeling can be performed to calculate the responseof the flaw as it passes beneath a single element of the array, as shownin FIG. 21. This two-dimensional analysis assumed a given platethickness, a conductivity 17.4 MS/m, a lift-off of 0.15 mm, and arectangular surface breaking notch. The position of the return conductorwas also set in the model. The transimpedance between the secondary onone side of the central conductor and the drive current was calculatedfor various positions beneath the sensor array and used to determine asignal-to-noise-ratio using the formula${S\quad N\quad R} = \sqrt{\left( \frac{m - m_{o}}{n_{m}} \right)^{2} + \left( \frac{p - p_{o}}{n_{p}} \right)^{2}}$

[0101] where m denotes the transimpedance magnitude, p denotes thetransimpedance phase, the subscript o denotes response from the originalunflawed material distant from the flaw, and n denotes the noise in theinstrument response. This noise is determined empirically for existingsensors and assumed to be constant as the geomtry of the sensor isvaried.

[0102] The simulation results of FIG. 21 illustrate how theprimary-to-primary distance can affect the response of the sensor as itpasses over a flaw. With the standard primary-to-primary distance, FIG.21 shows a large indication when the flaw is between the central drivewinding segments and the sensing element. There is also a significantpeak in the response when the flaw is nearly beneath the return leg ofthe primary winding and a minor peak above the outer conductor for thesecondary winding. As the primary-to-primary separation distance isincreased, the primary peak increases slightly and the peak associatedwith the return leg of the primary is reduced. This is desirable becausea larger signal is obtained from the flaw and the reduction in thedistant peak helps to reduce the appearance of “ghost” signals in scanimages, where multiple indications are shown for a single flaw. Theminor peak above the outer conductor for the secondary winding is alsoenhanced as the primary-to-primary distance is increased so that more ofthe signal is concentrated in the vicinity of the sensing secondaryelement, which again reduces the “ghosting” effect. An example of amodified sensor design is shown FIG. 22. In this sensor array, all ofthe sensing elements 76 are on one side of the central drive windings71. The size of the sensing elements and the gap distance 80 to thecentral drive windings 71 are the same as in the sensor array of FIG.20. However, the distance 81 to the return of the drive winding has beenincreased, as has the drive winding width to accommodate the additionalelements in the single row of elements.

[0103] In some applications, such as aircraft lap joint inspection forcracks or corrosion or weld inspection for stress or defects, it isdesirable to map or image the properties of the MUT across the entireregion of interest with a single scan pass and for extended distances.Raster scanning a single element sensor across the zone of interest anddown the length of the inspection region can provide a high resolutionimage of the MUT properties both across and along the inspection region,but is very time consuming. In contrast, longitudinal scanning with alinear array of sensing elements, which provides information about theMUT properties in the transverse direction, can be much more efficient.The number, size and location of the sensing elements in the arraydetermine the transverse resolution of the property image created by thearray across the inspection region. The scan speed and data acquisitionrate determine the resolution in the longitudinal, scan, direction. Whenthere are characteristic features of the MUT properties across theinspection region that indicate the quality of the region, the array ofsensing elements can be tailored for that particular type of inspection.

[0104] As an example, consider the inspection of a friction stir weld(FSW). The formation of an FSW is characterized by complex metal flowpatterns and microstructural changes. Three distinctly different majorzones can be typically identified as: (1) a dynamically recrystallizedzone (DXZ), or weld nugget, (2) a thermomechanical or heat- anddeformation-affected zone (TMZ), adjacent to the weld nugget on bothleading and trailing sides of the joint, and (3) a heat-affected zone(HAZ) (Arbegast, 1998; Ditzel, 1997). The two types of defects that havebeen noted in friction stir welds are: (1) tunnel defects within thenugget and (2) lack of penetration (LOP) (Arbegast, 1998). LOP existswhen the DXZ does not reach the backside of the weld due to inadequatepenetration of the pin tool. The LOP zone may also contain awell-defined cracklike flaw such as a cold lap, which is formed bydistorted but not bonded original faying, i.e., butt, surfaces. Thisoccurs as a result of insufficient heat, pressure and deformation.However, the LOP can be free of well-defined cracklike flaws, yet not betransformed by the dynamic recrystallization mechanism sincetemperatures and deformation in the LOP may not be high enough. Althoughit may contain a tight “kissing bond,” this second type of LOP defect isthe most difficult to detect with alternative methods such asphased-array ultrasonic or liquid penetrant inspection.

[0105] For an FSW, the quality of the weld or the joint between the basematerials can be determined from features in the measurements of theelectrical conductivity profile across the joint region, as described inmore detail in U.S. application Ser. No. 09/891,091, now abandoned, aswell as in U.S. application Ser. No. 10/046,925, filed Jan. 15, 2002,the entire contents of which are incorporated herein by reference. Forexample, planar flaws can appear as sharp reductions in the electricalconductivity and, for some alloys, the width of the peaks in theelectrical conductivity profile can provide a measure of the DXZ widthand LOP. Local reductions or dips in the electrical conductivity nearthe edges of the DXZ, as illustrated in FIG. 23, can also provideinformation about the quality of the weld. In order to inspect thesewelds, the sensor array needs to be wide enough to cover the entire weldregion. In addition, differences in the base material properties, suchas the electrical conductivity, can drastically affect the propertyprofile across the weld, so it is important to have sense elementsoutside the weld zone.

[0106] A sensor array embodiment suitable for FSW inspection is shown inFIG. 24. Here, most of the sensing elements 76 are located in a singlerow to provide the basic image of the material properties. A smallnumber of sensing elements 72 are offset from this row to create ahigher image resolution in this location, which is the location of a“dip” in electrical conductivity near the edge of the DXZ. In addition,several other sensing elements 96 and 98 are located a distance awayfrom the main grouping of sensing elements in order to obtainmeasurements of the base material properties of the plates being joined.Alternatively, the size of the elements in the different regions couldalso be varied. Other combinations or groupings of the sensing elementsare also within the scope of this description.

[0107] In one embodiment, the number of conductors used in the primarywinding can be reduced further so that a single rectangular drive isused. As shown in FIG. 25, a single loop having extended portions isused for the primary winding. A row of sensing elements 75 is placed onthe outside of one of the extended portions. This is similar to designsdescribed in U.S. Pat. No. 5,453,689 where the effective wavelength ofthe dominant spatial field mode is related to the spacing between thedrive winding and sensing elements. This spacing can be varied to changethe depth of sensitivity to properties and defects. Advantages of thedesign in FIG. 25 include a narrow drive and sense structure that allowsmeasurements close to material edges and non-crossing conductor pathwaysso that a single layer design can be used with all of the conductors inthe sensing region in the same plane. The width of the conductor 91farthest from the sensing elements can be made wider in order to reducean ohmic heating from large currents being driven through the drivewinding. However, this design has half the signal of the designs in FIG.18, FIG. 20, and FIG. 22.

[0108] In another embodiment, multiple rows of sensing elements areused. FIG. 26 shows a single rectangular drive winding 102 with sensingelements 110 and 112 outside of the drive winding and on either side ofthe extended portions of the rectangular drive. The distances 114 and116 between the sense elements and the drive winding are selected, asdescribed in U.S. Pat. No. 5,453,689, to provide a prescribed effectivedepth of penetration of the magnetic field into the MUT and a prescribedsensitivity to material properties or anomalies of interest. In anembodiment, the second row of sensing elements 112 is aligned with thefirst row of sensing elements 110 so that when scanning or surfacemounted the array sensing elements detect the same crack or anomalytwice as it move across or propagates across the sensor. To facilitatemeasuring the same response from sensing elements on either side of thedrive winding to an anomaly, the distances 114 and 116 should be madeequal. The current source connection 106 to the drive winding should becentered so that the distance to each of the extended portions of therectangular drive are the same. In another embodiment, shown in FIG. 27,the spacing 114 between one set of sensing elements and the drive isdifferent than for the spacing 116 for the sensing array on the oppositeside of the drive to provide two effective depths of sensitivity. Thiscan also be accomplished with the designs in FIG. 18, FIG. 20, and FIG.22. In another embodiment, shown in FIG. 28, the sensing elements 112are offset from the sensing elements 110 parallel to the extendedportions of the rectangular drive to improve coverage for scanning andimaging of material properties or anomalies. In a preferred embodiment,this offset distance is one-half the length of the sensing element thatis parallel to the extended portions of the rectangular drive.

[0109] In each of the embodiments illustrated in FIG. 26, FIG. 27, andFIG. 28, the sensing elements can be placed either within the drive oron either side of the drive. These sensing elements can be placed in thesame layer as the drive winding or on different layers. For sensingelements placed within the drive winding rectangle, the leads to thesensing elements must either be placed in a different layer than thedrive winding conductors and separated from the drive winding conductorsby a layer of insulation or the leads to the sensing elements need topass through the back of the sensor, out of the plane formed by thedrive windings. The use of flux cancellation leads, described earlier,is also preferred. An embodiment showing both rows of sensing elementsclose to one drive winding conductor is shown in FIG. 29. The return 104for the drive winding is placed on a second layer. In anotherembodiment, shown in FIG. 30, an active or passive electronic circuit120 is added at the opposite end of drive winding from the currentsource connection 106 to either amplify the current, reduce theself-inductance of the drive winding, reduce capacitive effects, orminimize thermal effects. In one embodiment, an active circuit is usedto alter the resonant frequency of the drive circuit.

[0110] In a related embodiment, the single rectangular drive with one ormore sensing elements is fabricated on a flexible substrate with a foamor other conformable or fluid support substrate. This substrate holdsthe sensor and allows it to be pressed against a curved or flat surfaceduring scanning to measure material properties or detect defects, asdescribed in U.S. application Ser. No. 09/946,146 filed Sep. 4, 2001,now abandoned, the entire teachings of which are incorporated herein byreference. This can be accomplished for the detection of cracks orfretting damage in engine disk slots, and the detection of cracks inbolt hole or other complex shaped MUT. The sensor can also be attachedto a rigid substrate that is flat or shaped to match the curvature of anMUT. The measurements can then be performed in a non-contact scanningmode or a permanently mounted mode.

[0111] Eddy current sensor arrays are well-suited for the inspection oflarge areas for materials characterization (e.g., coating thicknessmeasurements, shot peen quality assessment, and weld inspection), thedetection of surface breaking and subsurface flaws (e.g., cracks andinclusions), and the detection of hidden corrosion. These sensor arrays,shown for example in FIG. 1, FIG. 11, FIG. 20, and FIG. 24, have one ormore linear arrays of sensing elements oriented perpendicular to thescan direction. Then, a simple scan of the array provides a measurementimage of the material properties, either in the form of the rawtransimpedance magnitude and phase or in the form of effective materialproperties if processed with measurement grids. In contrast, the use ofsingle element or conventional eddy current sensors requires scanning intwo directions, which is more time consuming than a single directionscan but can provide higher resolution images than the linear array ofdiscrete elements.

[0112]FIG. 31 and FIG. 32 provide images showing distributedmicrocracks, small cracks and visible macrocracks in an aluminum bendingfatigue specimen. The images are taken with the sensor in two differentorientations to demonstrate the effect of the induced eddy currentorientation on the sensitivity to cracks. For these specimens, thedistributed small cracks are dominantly oriented perpendicular to theaxis of the specimen (parallel to the bending moment axis).Consequently, FIG. 32 shows the regions of microcracking moreprominently than FIG. 31.

[0113]FIG. 33 provides the “measurement grid” used to estimate theconductivity and lift-off from the transinductance magnitude and phasedata for each sensing element of the MWM-Array. For this grid, the twounknowns are the conductivity and lift-off. In this case, the modelassumes the aluminum layer is an infinite half space. The data shown inFIG. 33 is for a single channel of the MWM-Array from the scan in FIG.31.

[0114] An example subsurface defect detection application is theinspection of the C-130 flight deck chine plate for hidden corrosion.The corrosion typically occurs on the inaccessible backside of the platewhile the exposed surface of the chine plate may contain, with areas ofmanual material removal by grinding. The plate thickness between thereinforcing ribs (stiffeners) normally ranges between 0.043 and 0.047in.

[0115] An image of the plate thickness obtained from a scan with anMWM-Array is shown in FIG. 34. Another plate thickness image is shown inFIG. 35, with FIG. 36 showing the same data with a scale highlightinglow to intermediate corrosion loss regions. A measurement grid is usedto convert the magnitude and phase measurements at each sensing elementinto estimates of plate thickness and lift-off, where lift-off is theproximity of the sensor to the outer metal surface, includingcontributions from roughness and paint. The result is a lift-offcorrected image of the plate thickness. This permits scanning withoutpaint removal, which is essential for the chine plate inspectionapplication. Note that the numbers along the vertical axis in the imagescorrespond to channel numbers. Each channel covers a 0.1-in wide area.When the MWM-Array partly overhangs the edge of the chine plate, imagingof internal geometric features and material loss close to complexfeatures such as edges and integral stiffeners is possible. Materialloss on inaccessible surface around one of the fastener holes, of 15percent to 40 percent, is readily apparent from the image. Other workhas shown that surface corrosion on the accessible surface that wasmanually ground out is also detectable; in some cases 50 percent tonearly 100 percent of the material has been removed in an attempt toremove the corroded areas. One new capability provided by the use ofabsolute sensing elements with long linear drive segments is thereduction of edge effects. By making the sensing elements small, defectsand properties near and even at edges can be imaged.

[0116] Measurements performed on simulated corrosion test specimen havealso demonstrated the capability of the MWM-Array to quantify and locatehidden material loss. As an example, measurements were performed on atwo-layer test specimen simulating hidden corrosion in a lap joint,where the simulated material loss had a dome-shaped area machined out ofone of the layers. A plate of uniform thickness then covered the domedcutout region. Measurement scans with the MWM-Array were performed onboth sides of the plate so that the simulated material loss could be ineither the first layer, nearest the sensor, or the second layer,farthest from the sensor. Each plate had a nominal thickness of 1 mm(0.040-in) and was fabricated from an aluminum alloy.

[0117]FIG. 37 shows images of the corrosion loss in the 5 percent lossspecimens for loss in the first and second layer taken at a frequency of10 kHz. These scan images illustrate the high resolution imagingcapability of the MWM-Array and demonstrates its high sensitivity tomaterial loss of 5 percent, with apparent sensitivity to material lossbelow 1 percent and relative thickness resolution potentially to a smallfraction of a percent. Similar measurements were performed on higherloss samples, including 10 percent, 20 percent, and 30 percent loss.FIG. 38 and FIG. 39 show the responses of a single channel of theMWM-Array to material loss between two layers as the element is scannedacross the loss region. For first or second layer material loss, thenature of the MWM-Array response varies significantly with material losslocation. This variation of the response with position provides anindication of the layer in which the loss is occurring and also showsthat improper assumptions regarding the location of the corrosion lossmay result in errors in the material loss estimates. For corrosiondetection alone, this may not be important. However, erroneousassumptions will affect sensitivity and robustness, and, forprioritization based on actual material loss percentages, it is criticalto account properly for the material loss location. FIG. 40 shows acomparison of the measurements for the material loss in the first orsecond layers. There is good quantitative agreement between the twomeasurements, indicating that using an air calibration for the sensorand measurement grids based on a reasonable model for the response overthe MUT can provide a robust measurement procedure. Multiple frequenciescan also be used to estimate multiple unknowns, including paintthickness, first layer material loss, second layer material loss,conductivity of layers, gap thickness, and Alclad layer thickness. Also,the high resolution image produced by the MWM-Array permits (1)identification and estimation of stress concentrations (K factors) thatmay limit life, (2) characterization of exfoliation corrosion damage,and (3) remaining life/damage tolerance assessments.

[0118] MWM-Arrays also provide a capability to perform bi-directionalmagnetic permeability measurements in a scanning mode. FIG. 41 throughFIG. 44 provide images of the magnetic permeability for a broken tensilespecimen of 4340 low alloy steel. The MWM-Array was scanned across andalong the gage section of a specimen broken in a tensile test and thepermeability was measured at two frequencies, 158 kHz for FIG. 42 andFIG. 44 and 1 MHz for FIG. 41 and FIG. 43. In FIG. 41 and FIG. 42 theextended portions of the primary winding were oriented parallel to theloading axis. In FIG. 43 and FIG. 44 the extended portions of theprimary winding were oriented perpendicular to the loading axis. Thisillustrates the potential to map residual stress variations produced,for example by a hard landing, in parts fabricated from carbon and lowalloy steels. Notice that the permeability images at low and highfrequencies reveal stress changes with distance from the surface. A highresidual stress region near the fracture is indicated in the images ofFIG. 43 and FIG. 44. To create these images, a permeability/lift-offmeasurement grid was used, as shown in FIG. 45, assuming a knownconductivity and an infinite half-space (i.e., the steel layer isassumed to be infinitely thick). The relationship between permeabilityand stress is described in a technical paper titled “Residual andApplied Stress Estimation from Directional Magnetic PermeabilityMeasurements with MWM Sensors” submitted to the ASME Journal PressureVessels and Piping, the entire teachings of which are incorporatedherein by reference. Also, the MWM has demonstrated a capability toassess grinding process quality and detect carbide content and othermetallurgical and material features of interest. Since the lift-off ordistance between the sensing windings and the test material is beingmeasured through the measurement grids, the residual stress measurementcan be performed in a non-contact mode, which ensures that the sensorand probe assembly do not influence the stress distribution on thecomponent.

[0119] The MWM construct itself was also designed to have reducedsensitivity to its own temperature so that it could operate in elevatedtemperature environments. The temperature affects the conductivity ofthe winding conductors that, in turn, affects the current distributionin the conductors. The sensitivity to winding conductivity variationswith temperature is minimized by maintaining a sufficient gap betweenthe primary and secondary windings. Then, the transverse diffusion ofcurrents, in which the currents in the primary winding crowd out towardsthe winding surfacers, does not cause significant increases in inductivecoupling between the primary and secondary, as described in U.S. Pat.No. 5,453,689. This also permits the use of MWM sensors and sensorarrays to measure the temperature of components. Preferably, this isdone in a non-contact mode to minimize any perturbations in the thermalenvironment; in a contact mode, thermal heat transfer through the sensorand probe assembly could significantly affect the temperature of thecomponent and any treatment being performed.

[0120] In another embodiment, the sensors can be designed usingautomated tools incorporating layout rules for the conductor pathways.This tool takes input information for the dimensions and quantity of thedrive and sense elements and automatically draws the sensor layout usingrules for a sensor family, such as a single element MWM, an MWM-Array,or a Rosette. In one implementation, a Matlab script processes the inputinformation and passes it to AutoCad for the rendering the sensordesign.

[0121] Another application well-suited to conformable eddy currentsensor arrays is the permanent mounting of sensors indifficult-to-access locations. This provides an inspection capabilitythat safely supports life extension for aging structures and reducesweight and maintenance/inspection costs for new structures that requireboth rapid and cost effective inspection capabilities. In particular,continuous monitoring of crack initiation and growth requires thepermanent mounting of sensors to the component being monitored andseverely limits the usefulness of calibration or reference standards,especially when placed in difficult-to-access locations on aging or newstructures. Furthermore, in many difficult-to-access locations, theactual inspection is relatively short and the costly, time-consumingpart is the disassembly to permit access to the location or surfacepreparation to remove, for example, sealant layers. In one embodiment,the capability to measure stress, through permeability, is combined withpermanently mounted sensors to provide a contact or non-contact stressmeasurement capability.

[0122] Conventional eddy current designs are not ideal for permanentmounting. Conventional eddy-current techniques require varying theproximity of the sensor (or lift-off) to the test material or referencepart by rocking the sensor back and forth or scanning across a surfaceto configure the equipment settings and display. For example, for crackdetection the lift-off variations is generally displayed as a horizontalline, running from right to left, so that cracks or other materialproperty variations appear on the vertical axis. Affixing or mountingthe sensors against a test surface precludes this calibration routine.The probe-to-probe variability of conventional eddy-current sensorsprevents calibrating with one sensor and then reconnecting theinstrumentation to a second (e.g., mounted) sensor for the test materialmeasurements. These shortcomings are overcome with spatially periodicfield eddy-current sensors that provide absolute property measurementsand are reproduced reliably using micro-fabrication techniques.Calibrations can also be performed with duplicate spatially periodicfield sensors using the response in air or on reference parts prior tomaking the connection with the surface mounted sensor. The capability tocharacterize fatigue damage in structural materials, along with thecontinuous monitoring of crack initiation and growth, has beendemonstrated, as described in U.S. application Ser. No. 09/666,879, nowU.S. Pat. No. 6,657,429, and Ser. No. 09/666,524. This inspectioncapability is suitable for on-line fatigue tests for coupons and complexcomponents, as well as for monitoring of difficult-to-access locationson both military and commercial aircraft.

[0123] The surface mountable MWM-Rosette shown in FIG. 46 is just oneexample of a sensor design suitable for surface mounting on aircraft.The design of surface mountable MWM-Arrays includes three requirements:(1) the sensing footprint must be large enough to cover the region ofinterest within which cracks might initiate and propagate, (2) theresolution of the sensing elements must be sufficient to monitor growthrates and estimate crack length (if more than just detection isrequired, subelement crack length variations can be estimated from thesignal size as well), (3) at least one sensing element should be locatedin a region not likely to contain cracks during the inspection period.Three basic constructs for surface mounted sensors may be used: (1) theMWM-Rosette is designed for detection of cracks at fasteners as shown inFIG. 46, (2) the linear MWM-Array format shown in FIG. 47 can be used tomonitor crack initiation and growth along a linear feature, e.g., aradius in an aircraft structural, and (3) the linear array format shownin FIG. 48 can be used to detect cracks that propagate across a specificlocation within a structure member. Each of these designs can be locatedon an exposed surface or sandwiched between layers (e.g., skins).

[0124]FIG. 49 provides data from a fatigue test with an MWM-Rosettemounted around a hole in an aluminum dogbone specimen. Each channelnumber corresponds to an individual annular sensing element, withchannel 1 being closest to the fastener and channel 7 the furthest fromthe fastener. FIG. 50 shows a crack growth curve based on the data shownin FIG. 49 and known MWM-Array geometry. The conductivity drop in eachchannel occurs when the crack approaches the primary winding on theinner side of the sense winding.

[0125] Other types of sensing elements can also be used in these arrays.The small rectangular sensing elements 72 shown, for example in FIG. 2,could be super-conducting SQUID type sensors, Hall effect probes,magnetoresistive (MR) sensors, giant magnetoresitive (GMR) sensors, orwound eddy current sensor type coils. A representative sensor that usesa GMR sensor as a sensing element and a rotationally symmetricdistributed drive winding is shown in FIG. 51 and described in detail inU.S. application Ser. No. 10/045,650, filed Nov. 8, 2001, the entireteachings of which are incorporated herein by reference. For this drivewinding, the number of turns in each circular winding segment 30 isvaried to shape the field. Interconnections between each segment aremade with tightly wound conductor pairs 32 to minimize fringing fieldeffects. A GMR sensor 34, with feedback controlled coil, is placed atthe center of the concentric circular drive windings. Connections tothis hybrid sensing element are made with a tightly wound conductor pair36. Both the number of turns and the polarity of the windings (currentdirection) can be varied in the drive winding segments. In this case,there are two sets of drive windings which allows more than onefundamental spatial mode. The polarity of the connection determineswhich of the two current drive patterns (with different fundamentalspatial wavelengths) is excited. This provides two distinct field depthsof penetration conditions and permits improved multiple propertymeasurements for layered media.

[0126] Once the sensor response is obtained, an efficient method forconverting the response of the GMR sensor into material or geometricproperties is to use grid measurement methods. These methods map themagnitude and phase of the sensor response into the properties to bedetermined. The sensors are modeled, and the models are used to generatedatabases correlating sensor response to material properties. Themeasurement grids are two-dimensional databases that can be visualizedas “grids” that relate two measured parameters to two unknowns, such asthe conductivity and lift-off (where lift-off is defined as theproximity of the test material to the plane of the sensor windings). Forcoating characterization or for inhomogeneous layered constructs,three-dimensional grids (or higher order grids), called lattices (orhyper-cubes), are used. Similarly, a model for the GMR sensor withfeedback loop and circular drive windings was developed and used togenerate measurement grids, which were then used to interpret sensorresponse. Alternatively, the surface layer parameters can be determinedfrom numerical algorithms that minimize the least-squares error betweenthe measurements and the predicted responses from the sensor.

[0127] An advantage of the measurement grid method is that it allows forreal-time measurements of the absolute electrical properties of thematerial. The database of the sensor responses can be generated prior tothe data acquisition on the part itself, so that only table lookupoperation, which is relatively fast, needs to be performed. Furthermore,grids can be generated for the individual elements in an array so thateach individual element can be lift-off compensated (or compensated forvariation of another unknown, such as permeability or coating thickness)to provide absolute property measurements, such as the electricalconductivity. This again reduces the need for extensive calibrationstandards. In contrast, conventional eddy-current methods that useempirical correlation tables that relate the amplitude and phase of alift-off compensated signal to parameters or properties of interest,such as crack size or hardness, require extensive calibrations andinstrument preparation.

[0128] Several sets of measurements have been performed with acircularly symmetric shaped field magnetometer. These measurements usedthe GMR eddy current sensor with drive illustrated in FIG. 51. A simpleone-point air calibration method is used for all of these measurements.This means that the sensor response when over the test material wasnormalized by the sensor response in air, away from any conducting ormagnetic materials. The measurement results are then processed withmeasurement grids to provide absolute property measurements, such aselectrical conductivity, magnetic permeability, material thickness, andsensor proximity (lift-off). The absolute property measurementcapability eliminates the need for extensive, and in some cases any,calibration sets. Even if reference calibrations are performed, possiblyto improve the accuracy of the property estimation, only a singlecalibration material may be required. Air and reference part calibrationmethods have previously been described for square wave meanderingwinding constructs in U.S. Pat. No. 6,188,218, the contents of which arehereby incorporated in its entirety. The discrete segment Cartesian andcircular geometry sensors described herein can also be calibrated inthis fashion because the sensor response can be accurately modeled. Inprinciple, air calibrations in this context can be performed with anysensor whose response can be accurately modeled.

[0129]FIG. 52 shows the measurement grid for conductivity/lift-offmeasurements with three different materials, in the form of metalplates, over a range of lift-off values. Since both the conductivity andthe lift-off parameters vary over a relatively large range, theparameter values for this grid are chosen on a logarithmic scale. Thegrid cell area is a measure of the sensitivity of the measurement inthat region of the grid. The measurements are carried out at 12.6 kHz.Placing plastic shims between the sensor and the metal plates varied thelift-off. The three data sets follow lines of constant conductivity veryclosely. As listed in Table 1, the measured lift-off values were inexcellent agreement with the nominal values. Only the first 12 sets arelisted, due to the lack of sensitivity at higher lift-off values, asillustrated by the narrowing of the grid cells in FIG. 52.

[0130] The lowest value of the lift-off, 3.3 mm, corresponds tomeasurements with no shim, and is equal to the effective depth of thewindings below the surface of the sensor. This amount has been added tothe data in the last column, after having been estimated by taking theaverage of the difference between the magnetometer estimated values andthe measured shim thicknesses. This number is quite reasonable, giventhat the average depth of the grooves is on the order of 3 mm, and thatthe winding thickness, about 2 mm, is not considered by the model. Theconductivity data in Table 1 are also in good agreement with valuesreported in the literature. There appears to be an optimal range of thelift-off, 5-7 mm, where the estimated conductivity is most accurate.This is reasonable since sensitivity is lost at higher lift-offs, whilea close proximity to the sensor windings is also not desirable since theeffects of the non-zero winding thickness then become more significant.These conductivity results are also remarkable good considering thatthis measurement was carried out with no calibration standards and witha single air calibration point, the model for the sensor response isrelatively simple, and no empirical data have been used to determine thesensor response. If it is necessary to perform a very exact conductivitymeasurement, then a two-point reference part calibration is recommended,with the properties of the two reference parts (or the same part at twolift-off values) bracketing the properties of the unknown part. Theseresults confirm the validity of the model for this cylindricalcoordinate sensor. TABLE 1 Measurement results corresponding to FIG. 52.Conductivity [MS/m] Lift-off [mm] Nominal Data Cu Cu Lift- Set 110 Al6061 Al 2024 110 Al 6061 Al 2024 off [mm]  1 59.2 29.5 18.0 3.2 3.3 3.33.3  2 59.2 28.9 17.8 4.0 4.1 4.1 4.1  3 58.7 28.7 17.8 4.7 4.8 4.5 4.8 4 58.3 28.6 17.6 5.5 5.6 5.6 5.6  5 57.8 28.3 17.6 6.4 6.5 6.5 6.5  657.1 28.1 17.5 7.3 7.1 7.3 7.3  7 55.7 27.4 17.3 7.9 8.0 8.0 8.0  8 56.127.5 17.4 8.7 8.9 8.8 8.8  9 54.3 26.8 17.1 9.4 9.5 9.4 9.4 10 55.2 27.017.2 10.2 10.3 10.3 10.2 11 53.5 26.4 17.0 10.8 10.9 10.9 10.9 12 53.026.3 16.7 11.7 11.7 11.7 11.7

[0131] Another set of measurements illustrates the GMR magnetometercapability to detect material flaws in a thick layer of metal. Thesemeasurements were carried out by performing scans over a set ofstainless steel plates. One plate had a 25 mm long, 0.4 mm wide, and 2.4mm depth slot to simulate a crack. The crack is not modeled explicitly,but its presence is usually manifested by a local reduction in the valueof the measured conductivity. In some cases, depending on its depth andposition below the surface, it may appear as a local change in thelift-off. Several sets of scans were made with stainless steel platesarranged to simulate a crack at the upper surface, nearest the sensor, acrack 3.2 mm below the upper surface, and a crack 7.2 mm below thesurface. The image generated by one scan, with the slot at the surface,is shown in FIG. 53. This image shows the conductivity, normalized byits value away from the crack. The crack signal is very strong, with theconductivity decreasing more than 3% near the crack position. The doublehump signature of the crack is characteristic of the effect cracks haveon the signal of imposed-periodicity eddy current sensors. The inducedcurrent density mirrors the current density of the drive, and as aconsequence, the disruption caused by the crack is greatest when it isdirectly below, and perpendicular, to the primary winding nearest to thesensing element. For deeper cracks, near the crack, the measuredconductivity is actually higher. This is because the phase of theinduced eddy currents changes with depth. With the crack positioned 7.2mm below the surface it interrupts eddy currents that are flowing in adirection opposite to the surface eddy currents, thereby increasing themagnetic field at the sensor. A consequence of this effect is that thereis a characteristic depth, near π/2 skin depths, where a crack wouldcause no change in the conductivity.

[0132] A GMR sensor can be placed in a feedback configuration with asecondary winding, as shown in FIG. 54. In this way the magnetic fieldat the GMR sensor remains nearly constant during operation, eliminatingthe effect of the nonlinear transfer characteristic, while maintainingsensitivity at low frequencies. The magnitude of the current in thesecondary winding is taken as the output signal, and since therelationship between this current and the magnetic field for an air-corewinding is linear, so is the transfer characteristic of the entirehybrid sensor structure. The magnetic field magnitude that this hybridGMR sensor can measure is limited only by the magnitude of the fieldthat the secondary winding can produce, which can be orders of magnitudehigher than the saturation field of the GMR sensor. This dramaticallyincreases the dynamic range of the GMR sensor and makes its use far morepractical than in alternative implementations with permanent magnets orelectromagnets that provide a constant bias.

[0133] Another benefit of the feedback configuration is temperaturestability. Since the measured quantities are currents in the windings,which are directly related to the magnetic fields, temperaturedependence of the GMR sensor on winding resistance, etc. has no effecton the magnetometer response. This is critical since temperaturevariations have limited reproducibility and limit the use of manycommercially available eddy current arrays. Goldfine and Melcher (U.S.Pat. No. 5,453,689) solved the temperature sensitivity problem forinductive sensing elements by maintaining a gap between drive andsensing windings. Temperature stability is a key to the practical use ofGMR sensors as well.

[0134] Another advantage of the feedback connection is for biasing theGMR sensor. Biasing the GMR sensor to the appropriate operating point isaccomplished simply by adding an appropriate DC voltage offset at theinput of the gain stage. This is much better than the alternativebiasing methods described earlier, since correct biasing is maintainedeven if the position of the GMR sensor with respect to the bias sourcechanges, which would not be true for biasing with a constant fieldsource. This eliminates the need for complex alignment methods, sincebiasing at the correct level is automatic with the appropriate choice ofcircuit components. As a result, this feedback configuration providesthe same sensitivity of a GMR sensor by itself while maintaining alinear transfer characteristic and a wider dynamic range.

[0135] The position of the GMR elements within the feedback coil, andthe position of the feedback coil within the primary winding can also beadjusted. FIG. 55 illustrates that one or more GMR sensors 84 can besurrounded by a feedback coil 82 and placed at the center of a drivewinding 80. The use of multiple GMR sensors within the footprint of thedrive winding promotes imaging of material properties when the array isscanned in a direction perpendicular to the row of GMR sensors. The useof a single feedback coil and multiple GMR sensor elements eliminatescross-talk between elements, which may occur if each GMR element has itsown feedback coil, and also simplifies the drive circuitry for thesensor array. FIG. 56. shows a similar array with the row of GMRelements 84 and feedback coil offset so that it is closer one side ofthe primary winding than the other. This results in an asymmetricresponse when the array is scanned over a flaw since the array is moresensitive to the effects of the flaw when it passes beneath the nearerportion of the primary winding. Similarly, sensing elements can beplaced outside of the drive winding, as illustrated in FIG. 57, wherethe row of sensor elements 84 is far from the drive winding 80 while asecond row of sensors 86 is near the drive winding. An advantage of thisconfiguration is that any connection leads to the sensing elements doesnot have to pass over the conductors of the drive winding, which helpsto minimize parasitic responses.

[0136] The inventions described here relate to methods and apparatus forthe nondestructive measurements of materials using sensors that applyelectromagnetic fields to a test material and detect changes in theelectromagnetic fields due to the proximity and properties of the testmaterial. Although the discussion focused on magnetoquasistatic sensors,many of the concepts extend directly to electroquasistatic sensors aswell.

[0137] While this invention has been particularly shown and describedwith references to preferred embodiments thereof, it will be understoodby those skilled in the art that various changes in form and details maybe made therein without departing from the scope of the inventionencompassed by the appended claims.

[0138] The following references are incorporated herein by reference intheir entirety:

[0139] Arbegast, W. J., and Hartley, P. J. (1998), “Friction Stir WeldTechnology Development at Lockheed Martin Michoud Space, Systems—AnOverview”, 5^(th) International EWI Conference on Trends in WeldingResearch, 1-5 June, 1998, Pine Mountain, Ga.

[0140] Auld, B. A. and Moulder, J. C. (1999), “Review of Advances inQuantitative Eddy-Current Nondestructive Evaluation,” Journal ofNondestructive Evaluation, vol. 18, No. 1.

[0141] Ditzel, P., and Lippold, J. C. (1997), “Microstructure EvolutionDuring Friction Stir Welding of Aluminum Alloy 6061-T6”, Edison WeldingInstitute, Summary Report SR9709.

[0142] The following references are also incorporated herein byreference in their entirety:

[0143] 1. Navy Phase I Proposal, titled “Wireless Communications withElectromagnetic Sensor Networks for Nondestructive Evaluation”, Topic#N01-174, dated Aug. 13, 2001.

[0144] 2. Air Force Phase I Proposal, titled “Three-Dimensional MagneticImaging of Damage in Multiple Layer Aircraft Structures”, Topic#AF02-281, dated Jan. 14, 2002.

[0145] 3. Final Report submitted to FAA, titled “Crack DetectionCapability Comparison of JENTEK MWM-Array and GE Eddy Current Sensors onTitanium ENSIP Plates”, dated Sep. 28, 2001, Contract#DTFA03-00-C-00026, option 2 CLIN006 and 006a.

[0146] 4. Technical Paper titled “Surface Mounted and Scanning PeriodicField Eddy-Current Sensors for Structural Health Monitoring”, presentedat the IEEE Aerospace Conference, March 2002.

[0147] 5. Technical Paper titled “Corrosion Detection and PrioritizationUsing Scanning and Permanently Mounted MWM Eddy-Current Arrays”,presented at the Tri-Service Corrosion Conference, January 2002

[0148] 6. Technical Paper titled “Shaped-Field Eddy Current Sensors andArrays”, presented at the SPIE Conference, March 2002.

[0149] 7. Technical paper titled “Residual and Applied Stress Estimationfrom Directional Magnetic Permeability Measurements with MWM Sensors,”submitted to ASME Journal Pressure Vessels and Piping.

[0150] 8. Technical paper titled “MWM-Array Characterization and Imagingof Combustion Turbine Components,” EPRI International Conference onAdvances in Life Assessment and Optimization of Fossil Power Plants,Orlando, Fla.; March 2002.

[0151] 9. Presentation slides “Fatigue Test Monitoring and On-AircraftFatigue Monitoring Using Permanently Mounted Eddy Current SensorArrays,” USAF ASIP Conference, Williamsburg, Va., December 2001.

[0152] 10. Technical presentation slides “Condition Assessment of EngineComponent Materials Using MWM-Eddy Current Sensors,” ASNT FallConference, Columbus, Ohio; October 2001.

[0153] 11. Technical presentation slides “High-Resolution Eddy CurrentSensor Arrays with Inductive and

[0154]  Magnetoresistive Sensing Elements,” ASNT Fall Conference,Columbus, Ohio; October 2001.

[0155] 12. Technical presentation slides “Surface Mounted MWM-EddyCurrent Sensors for Structural Health Monitoring,” ASNT Fall Conference,Columbus, Ohio; October 2001.

[0156] 13. Technical paper and presentation slides titled “HighThroughput, Conformable Eddy-Current Sensor Arrays for Engine DiskInspection including Detection of Cracks at Edges and in Regions withFretting Damage,” NASA/FAA/DoD Conference on Aging Aircraft, Kissimmee,Fla.; September 2001

[0157] 14. Technical paper and presentation slides titled“High-Resolution Eddy Current Sensor Arrays for Detection of HiddenDamage including Corrosion and Fatigue Cracks,” NASA/FAA/DoD Conferenceon Aging Aircraft, Kissimmee, Fla.; September 2001.

[0158] 15. Technical paper titled “Flexible Eddy Current Sensors andScanning Arrays for Inspection of Steel and Alloy Components,” 7^(th)EPRI Steam Turbine/Generator Workshop and Vendor Exposition, Baltimore,Md.; August 2001.

[0159] 16. Technical paper titled “Conformable Eddy-Current Sensors andArrays for Fleet-wide Gas Turbine Component Quality Assessment,” ASMETurbo Expo Land, Sea & Air, New Orleans, La.; June 2001.

[0160] 17. Technical presentation slides titled “Friction Stir Weld LOPDefect Detection Using New High-Resolution MWM-Arrays and MWMEddy-Current Sensors,” Aeromat 2001 Conference; June 2001.

[0161] 18. Technical paper titled “Applications for Conformable EddyCurrent Sensors including High Resolution and Deep Penetration SensorArrays in Manufacturing and Power Generation,” ASME 7^(th) NDE TopicalConference, San Antonio, Tex.; 2001.

[0162] 19. Technical paper titled “Surface Mounted Periodic FieldCurrent Sensors for Structural Health Monitoring,” SPIE Conference:Smart Structures and Materials NDE for Health Monitoring andDiagnostics, Newport Beach, Calif.; March 2001.

[0163] 20. Technical paper and presentation “Scanning and PermanentlyMounted Conformable MWM Eddy Current Arrays for Fatigue/CorrosionImaging and Fatigue Monitoring,” USAF ASIP Conference, San Antonio,Tex., December 2000.

[0164] 21. Technical presentation slides “Inspection of Gas TurbineComponents Using Conformable MWM Eddy-Current Sensors,” ASNT FallConference, Indianapolis, Ind.; November 2000.

[0165] 22. Technical paper titled “Anisotropic Conductivity Measurementsfor Quality Control of C-130/P-3 Propeller Blades Using MWM Sensors withGrid Methods,” Fourth DoD/FAA/NASA Conference on Aging Aircraft, St.Louis, Mo.; May 2000.

[0166] 23. Technical paper titled “Surface-Mounted Eddy-Current Sensorsfor On-Line Monitoring of Fatigue tests and for Aircraft HealthMonitoring,” Second DoD/FAA/NASA Conference on Aging Aircraft, August1998.

[0167] 24. Technical paper titled “Early Stage Fatigue Detection withApplication to Widespread Fatigue Damage Assessment in Military andCommercial Aircraft,” First DoD/FAA/NASA Conference on Aging Aircraft,Ogden, Utah, June 1997.

[0168] 25. Technical paper “Combustion Turbine Blade CoatingCharacterization Using a Meandering Winding Magnetometer,” ASNT FallConference, 1994.

What is claimed is:
 1. A test circuit comprising: a primary winding ofparallel conducting segments having extended portions including at leastone central conductor and at least one return conductor positioned oneither side of the central conductor to impose a magnetic field in atest material when driven by an electric current; a plurality of senseelements for sensing the response of the test material to the imposedmagnetic field, each sensing element positioned between the extendedportions of the primary winding, the sense elements being aligned withone another to sense the response at incremental areas along a pathparallel to the extended portions of the primary winding, and havingseparate output connections.
 2. A test circuit as claimed in claim 1wherein the distance between the central conductors and returnconductors are selected to align with features of a component beingtested.
 3. A test circuit as claimed in claim 1 including two centralconductors and two return paths symmetrically located on either side ofthe central conductors.
 4. A test circuit as claimed in claim 3 whereinthe distance between the central conductors and return conductors areselected to align with features of a component being tested.
 5. A testcircuit as claimed in claim 1 further comprising a second plurality ofsense elements aligned with one another to sense the response atincremental areas along a path parallel to the extended portions of theprimary winding, and having separate output connections.
 6. A testcircuit as claimed in claim 5 wherein each individual sense element inthe first plurality of sense elements is aligned with a sense element inthe second plurality of sense elements in a direction perpendicular tothe extended portions of the primary winding.
 7. A test circuit asclaimed in claim 5 wherein the sense elements in the first plurality ofsense elements is offset in a direction parallel to the extendedportions of the primary winding from the sense elements in the secondplurality of sense elements.
 8. A test circuit as claimed in claim 7wherein the offset distance is one-half of the length of a sensingelement.
 9. A test circuit as claimed in claim 5 wherein the distancesfrom the first plurality of sense elements and the second plurality ofsense elements to the central conductor are equal.
 10. A test circuit asclaimed in claim 9 wherein a differential measurement is taken between asense element in the first plurality of sense elements and a senseelement in the second plurality of sense elements.
 11. A test circuit asclaimed in claim 1 wherein the sensing elements and the centralconductor are in the same plane.
 12. A test circuit as claimed in claim1 wherein the location of the sense elements is non-uniform in thedirection parallel to the extended portions of the primary winding. 13.A test circuit as claimed in claim 1 wherein the primary winding andsense elements are fabricated onto a flexible substrate.
 14. A testcircuit as claimed in claim 1 further comprising a balloon filled with afluid to maintain contact between the test circuit and a surface undertest of the test material.
 15. A test circuit as claimed in claim 14wherein the balloon is attached to a shuttle and the shuttle is shapedto approximately match the shape of the material under test.
 16. A testcircuit as claimed in claim 15 further comprising a removable cartridgethat permits rapid replacement of the sensor and balloon components. 17.A test circuit as claimed in claim 14 wherein the surface under test isinside a bolt hole.
 18. A test circuit as claimed in claim 14 whereinthe surface under test is inside an engine disk slot.
 19. A test circuitas claimed in claim 1 wherein the conductivity and proximity of thesensor to the surface are measured to detect cracks.
 20. A test circuitas claimed in claim 1 wherein the proximity is measured at each sensingelement to determine surface roughness.
 21. A test circuit as claimed inclaim 1 wherein each sensing element response is used for healthmonitoring or condition assessment.
 22. A test circuit as claimed inclaim 1 wherein the primary winding and sense elements are fabricatedonto a rigid substrate.
 23. A test circuit as claimed in claim 1 whereinthe sensor is not in contact with a surface under test of the testmaterial.
 24. A test circuit as claimed in claim 1 wherein at least oneof the sense elements includes a magnetoresistive sensor.
 25. A testcircuit as claimed in claim 1 wherein the at least one of the senseelements includes a giant magnetoresistive sensor. 26 A test circuit asclaimed in claim 25 further comprising a secondary coil that surroundsthe giant magnetoresistive sensing element.
 27. A test circuit asclaimed in claim 26 wherein the secondary coil is in a feedbackconfiguration.
 28. A test circuit as described in claim 1 wherein thesensor response to a flaw is determined in advance, this response beingused to construct a filter, and the filter being applied to a sensorresponse to search for indications likely to be the flaw of interest andto suppress responses unlikely to be that flaw.
 29. A test circuit asdescribed in claim 1 wherein a single encoder is used to record theposition of the array in the scan direction during scanning.
 30. A testcircuit as described in claim 1 wherein a template is used to alignincremental scans of the sensor to construct an image of a wide area.31. A test circuit as described in claim 1 wherein an automated scanneris used to move the sensor across a surface of the material under test.32. A test circuit as described in claim 1 wherein modular fixtures withposition encoders are used to permit manual scanning of complex parts.